**Core Concept**
The question is testing the student's knowledge of bacterial capsules, specifically the unique polypeptide capsule of Bacillus anthracis. A bacterial capsule is a complex of polysaccharides or polypeptides that surrounds the cell wall and provides protection against phagocytosis.

**Why the Correct Answer is Right**
Bacillus anthracis has a polypeptide capsule composed of a repeating unit of anthranilic acid and glutamic acid, which is responsible for its virulence. The capsule is encoded by the pag gene and is essential for the bacterium's ability to cause disease. The polypeptide capsule of Bacillus anthracis is a key virulence factor that protects the bacterium from being phagocytosed by host immune cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Corynebacterium diphtheriae has a toxin that causes diphtheria, but it does not have a polypeptide capsule.
**Option B:** Clostridium welchii (also known as Clostridium perfringens) has a polysaccharide capsule, but it is not a polypeptide capsule.
**Option C:** Staphylococcus aureus has a polysaccharide capsule, but it is not a polypeptide capsule.

**Clinical Pearl / High-Yield Fact**
The polypeptide capsule of Bacillus anthracis is a key virulence factor that makes it resistant to phagocytosis, and this is a critical aspect of its pathogenesis. Understanding the unique characteristics of this capsule is essential for diagnosing and treating anthrax infections.

**✓ Correct Answer: D. Bacillus anthracis**